Many people have hearing impairments that decrease their quality of life. Most hearing impairments may be classified as one of two kinds, conductive or sensorineural. Conductive hearing losses are typically caused by a malfunction of the middle ear which interferes with the acoustic transmission of sound to the sense organ of the ear. A simulation of this kind of hearing loss is the reduced level of sound a person experiences when wearing ear plugs. The person's auditory processing system functions, but less than all of the sound is conducted to the sensory portions of the ear so that everything sounds quieter. In other cases the incoming sounds may be mechanically filtered by a frequency selective process. Generally, if a listener with a conductive loss is allowed to adjust the gain of a speech signal to his most comfortable level, speech intelligibility is almost normal.
Sensorineural hearing losses refer to an abnormality of the sense organ, the auditory nerve, or both. In these impairments, significant speech degradation persists despite adjustments to gain. Recruitment of loudness is one type of sensorineural impairment that affects the sense organ.
Loudness is an aspect of the sensation obtained by listening directly to a sound and is measured by the responses of a human observer. Intensity, on the other hand, is related to the power of the acoustic signal as measured by instruments. Loudness perception, unlike intensity, varies from person to person and with frequency. With recruitment of loudness, the loudness sensation of a tone grows more rapidly with an increase in physical intensity than it does in the normal ear.
Recruitment of loudness has the effect on speech perception of expanding the difference in perceived loudness between high amplitude vowels and low amplitude consonants. This effectively gives high frequency attenuation even if a listener's impairment does not become greater at high frequencies. With recruitment of loudness, the impaired subject has a reduced dynamic range of hearing that causes some conversational speech to fall below the subject's elevated threshold of hearing. It is often especially pronounced in the high frequency region where much of the information needed for consonant recognition is contained. If sufficient amplification to boost the high frequencies above the subject's threshold is provided, higher amplitude consonants would reach or exceed the discomfort level.
The phenomena described for recruitment of loudness are similar to those of speech masked by noise or other sounds. A sound is masked when it cannot be heard due to the presence of another sound. When a tone is just below the level of a masking noise it sounds very faint, but with just a small increase in its intensity, the loudness of the tone can be increased greatly. The phenomenon of the effects of a masker appearing beyond the frequency band of the masker is termed spread of masking. A person with sensorineural hearing loss will experience a greater than normal spread of masking which leads to masking between individual speech components.
The effects of masking have been studied for sinusoids and narrowband noise makers. Each masker can mask a region of the spectrum. The shape of the region differs for persons with sensorineural hearing impairments in direct relation to the amount of spread of masking. When more than one masker is present, the masking effects add whether the maskers are nonoverlapping, partially overlapping or totally overlapping.
Recruitment has not been successfully treated with currently available hearing aids. Typical hearing aids primarily amplify sounds so that the unaffected portions of the sense organ can be stimulated. The types of distortions associated with recruitment are often made worse with straight amplification. Accordingly, it will be appreciated that it would be highly desirable to have a signal processing apparatus and method that is nonlinear.
Amplication with some form of amplitude limiting has been used in hearing aids to bring speech and other sounds within the subject's reduced dynamic range of hearing. These techniques include linear amplification with automatic gain control, single channel compression where overall levels are compressed, and multichannel compression where compression is performed separately in different frequency regions. Each of these techniques have operated directly on the speech waveform and achieved limited success. Accordingly, it will be appreciated that it would be highly desirable to have a signal processing method that gives satisfactory results without operating directly on the speech waveform.
The perception of sound by persons having recruitment has been described as being equivalent to listening through a volume expander followed by an attenuator. A system employing amplitude expansion and attenuation has been used to simulate recruitment of loudness. Therefore, for compensation of recruitment, compression plus equalization was applied. Various types of compression systems have been developed including wideband and multiband compression. Multiband syllabic compression systems reduce the variation in speech level in each frequency band according to the subject's reduced dynamic range in that band. Single channel (wideband) systems process the entire speech signal on the basis of overall level. Although wideband processing cannot match a person's hearing profile as well as multiband processing, wideband processing does not distort the short term spectral shape.
The wideband and multiband compression systems mostly use digital or analog filters along with equalization gain. With these systems, the parameters remain constant over time, regardless of the input conditions. Linear amplification minimizes distortion and, with the use of automatic gain control, these systems can cause speech to remain below the subject's threshold of discomfort. However, automatic gain control systems, even with frequency-dependent gain, cannot adjust quickly to input transients and may cause some components to fall below threshold if high amplitude components are present.
In the past, both linear and compressive systems used parameters that remained fixed with time. Compressive systems did not change with input level and automatic gain control systems responded too slowly to input changes.
Multiband filter compression distorts the short-term spectral shape. Prior systems also ignored the spread of masking phenomenon. Accordingly, it will be appreciated that it would be highly desirable to have an apparatus and method that takes into account the spread of masking phenomenon and which adjusts quickly to transients.